The present invention relates generally to a light emitting device and a method of manufacturing the light emitting device and, more particularly, to a structure of the light emitting device such as a light emitting diode and a semiconductor laser and a manufacturing method thereof.
Known hitherto as a practical material of the light emitting device is a gallium nitride system compound semiconductor of gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN).
The following are explanations of a structure of the light emitting diode, a manufacturing method thereof and problems inherent therein by way of one example of the light emitting device manufactured by use of the above materials with reference to FIGS. 7A, 7B, 8A and 8B.
FIG. 7A is a sectional view illustrating the light emitting diode having a heterojunction, wherein a plurality of grown layers are formed based on an epitaxial growth. Further, FIG. 7B shows a profile of N-type impurities in the light emitting diode shown in FIG. 7A, wherein the axis of abscissa indicates an impurity concentration of the N-type impurity while the axis of ordinates indicates a distance from the underside of a substrate, corresponding to FIG. 7A.
The prior art light emitting diode is, as illustrated in FIG. 7A, constructed of stacking of grown layers such as a buffer layer 112 (a first GaN layer) composed of amorphous GaN, a spacer layer 113 (a second GaN layer) composed of monocrystalline GaN, a high-concentration N-type layer 114 (a third GaN layer) doped with the N-type impurity at a high concentration, an active layer 115 (an InGaN layer) composed of InGaN, an AlGaN layer 116 doped with a P-type impurity, and a contact layer (a fourth GaN layer) doped with the P-type impurity at a high concentration on the surface of a substrate 111 composed of sapphire and SiC.
The impurity concentration of the N-type impurity is on the order of 1-5.times.10.sup.18 atoms.cndot.cm.sup.-3 in the third GaN layer. In other layers, the impurity concentration of the N-type impurity is on the order of 1.times.10.sup.15 atoms.cndot.cm.sup.-3 defined as a background level. Further, these respective grown layers are formed by changing temperatures and sorts of gasses introduced thereinto, which involves the use of a vapor phase growth method such as a MO-CVD (Metal Organic Chemical Vapor Deposition) method. In the configuration given above, the second GaN layer 113 is formed by introducing hydrogen as a carrier gas, and ammonia (NH.sub.3) and TMGa (trimethyl gallium) as raw gases into the reaction chamber at 1000.degree. C.-1100.degree. C., and thereafter the third GaN layer 114 is so formed as to be doped with the N-type impurity at a high concentration by further supplying SiH.sub.4 (silane gas) while consecutively introducing the above gases. Incidentally, it is desirable that the second GaN layer has a thickness of 0.01 .mu.m or larger, and the third GAN layer has a thickness of 0.1 .mu.m.
By the way, the second GaN layer 113 is not functionally necessary essentially. Namely, if the third GaN layer doped with the N-type impurity at the high concentration is provided on the first GaN layer functionally provided as the buffer layer, the operation required of the light emitting device is to be performed. When a high-concentration monocrystalline GaN layer is provided on the surface of an amorphous GaN layer, however, as shown in, e.g., FIGS. 8A and 8B, a pin hole 211 is produced in the surface of the first GaN layer 112, or an abnormal growth 212 of the GaN layer with a dopant being a core, might occurs in some cases. Accordingly, in the prior art light emitting diode, the monocrystalline second GaN layer 113 is provided as a spacer layer on the surface of the first GaN layer 112, and subsequently the fourth GaN layer 114 doped the N-type impurity at a high concentration is formed.
As described above, in the semiconductor device using the prior art gallium nitride system compound semiconductor, when forming a high-concentration N-type layer under the active layer functioning as a light emitting layer, an undoped monocrystalline GaN layer is previously provided as a spacer layer beneath the high-concentration N-type layer in order to enhance the crystallinity thereof. Although the vapor phase growth method such as the MO-CVD method and so on for forming those respective grown layers, there must be formed the originally functionally unnecessary layer exhibiting a low growth velocity, and this results in a decreases in terms of throughput. Further, as the functionally unrequited layer is to be formed, the thickness of the entire light emitting device is to increase. When the layer thickness increases, a distortion quantity of each grown layer enlarges due to a lattice unjointed state, with the result that the functionally necessary grown layers must be deteriorated.